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Discussion Starter #1
I'm in the process of adding an extra extension speaker jack to a 25-watt combo using the detachable leads used for the amp's 8-ohm internal speaker. I'd need this extra jack to match up to parallel 8-ohm inputs on a g-flex 212.

Thing is, does the positive or negative go to the sleeve (grounded) tab ?

Is there a general rule-of-thumb for this ?

Will the extra cable length it'd take from the new jack to one side of the 212 have any adverse effect ?

Also, if I use the amp's factory extension jack (8-ohm) only, will that hurt anything ?
 

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"It's always something..."

Electric I said:
I'm in the process of adding an extra extension speaker jack to a 25-watt combo using the detachable leads used for the amp's 8-ohm internal speaker. I'd need this extra jack to match up to parallel 8-ohm inputs on a g-flex 212.

Thing is, does the positive or negative go to the sleeve (grounded) tab ?

Is there a general rule-of-thumb for this ?

Will the extra cable length it'd take from the new jack to one side of the 212 have any adverse effect ?

Also, if I use the amp's factory extension jack (8-ohm) only, will that hurt anything ?
A speaker is an AC device so + and - don't mean much. What IS important is phasing! This means that when you have more than one speaker you want the cones to move in and out together, pushing the air in phase rather than having some cones cancelling the net effort.

A speaker usually has its termnals marked either + or - or maybe a red dot on one. That way if you think of the terminals as pos or neg you can wire up extra speakers and always be in phase.

Here's the catch: This can ONLY be trusted with speakers of the same make and model. There is NO general rule of thumb for all speakers as to which terminal gets the red dot!

There's an easy way to check. If you take any small flashlight or 9 v battery and hook it across the speaker terminals the cone will immediately move out or in. As long as you're consistent about which terminal moves which way when positive you'll always be in phase.

Guitar amp manufacturers prefer not to think that someone might use a different speaker brand than they use! :) So they don't worry about it when they offer an extension speaker jack. They just wire it in parallel with the main jack. The sleeve is normally ground (most often to the chassis) and the tip is the "hot" lead.

Now, it's also different for tube amps and transistor amps. Tube amps often will wire the ext jack as a shorting jack for the ground path of the main speaker out. This means that if you use only the main speaker it is grounded through the extension jack but if you plug something into that extra jack it will put both speakers in series. Look at any old Fender or Ampeg schematic and you'll see what I mean.

This means that with an extra speaker the output tubes see a higher load rather than a lower (from being paralled) one.

Speaker loads can be treated as resistors like Ohm's Law if you want to use combinations. This means that speakers in series add their load and speakers in parallel will share it. Equal load share equally, which means two 8 ohm speakers in parallel will total a 4 ohm load. An 8 and a 16 in parallel don't calculate so easily and you'll need that Ohm's Law formula.

Power shares the same way in a combo of speakers. Two 8 ohm speakers together will each see half the power, whether in series or parallel. If one is 4 and the other 8 then the smaller 4 ohm speaker will "hog" the power big time! If they both are rated at 50 watts and you're running 100 watts from the amp the 8 ohm speaker will see much less than its share and the 4 ohm speaker will probably end up "kippered" from the overload.

Solid state amps can be tricky. If you add more parallel speakers the load will get smaller and smaller. Transistor circuits don't have a specific best speaker load and an output transformer with taps for 4, 8 16 or whatever. They simply put out more power (current) as the load gets smaller. The designer figures out a limit. He calculates that at a lower limit (maybe 4 ohms?) the transistors are handling their maximum rated power. If the load gets even smaller then they will try to pump too much power and will burn out!

That's why when you look at the back of a solid state head you may see "100 watts at 4 ohms. DO NOT RUN AT A LOWER SPEAKER LOAD!"

As for cable length for hookups - this is all hifi marketing crap designed to take advantage of folks that aren't very technical! Use the usual clear plastic stuff that looks like lamp cord wire that you can buy cheap at Home Depot for anything up to a few hundred watts.

Wire used for speaker hookups looks like a very low resistance to the circuit. Like any resistance it will cause losses. What those hifi "pseudo-techies" don't know or don't tell you is that in most cases with reasonable sized wire and runs under a hundred feet the total resistance/losses is "mice nuts". You'd need a special ohmmeter to measure it with the little bit needed to add a speaker jack.

If you want to buy the special stuff the guys are selling at the hifi store I guarantee you'll not need anything special to measure how much the price hurts!

What's more, that little bit of speaker wire resistance is just that, a resistance. Resistance in the speaker circuit is NOT a tone thing but just a LOSS thing!

If you run into anyone who claims he can hear the difference with different kinds of speaker wire then I suggest you try him with a blindfold test! Ask him to check the colour of your psychic aura while he's at it.

Hope this helps. You didn't mention what kind of amp you'e working with, whether tube or solid state. Or what speaker impedances are involved. Still, this should help you figure it all out.

You mentioned at the end that the amp already has an "8 ohm only" extension jack. That means that the amp is designed to run safely ONLY with an 8 ohm load at the output. You can screw around all you want with extra speakers and cabinet combinations but the amp doesn't know or care. It just knows that it needs an 8 load at that output, no matter what combo you used to make it.

Maybe you better let us know what kind of amp you're talking about.
 

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Discussion Starter #3
Thanks a ton for the clear & concise info, good sir.

I'm an electrician by trade & am versed in ohm's law & such, but as someone just getting into tube amp circuitry, I definitely have a lot to learn.

The amp in question is a Johnson Loredo 25-watt 110 combo. 2X12ax7 / 2XEL84

http://www.johnsongtr.com/Loredo-Tube.1014.0.html

The plan is to convert this amp into a low-wattage recording amp. Research has led me to believe that London Power's PS-Box is the way to go, along with their bias mod kit for maximum serviceability. I do plan on buying a tube amp book/dvd or two, so suggestions welcome.

I've already tinkered with the amp by swapping the budget stock speaker with an Eminence Ragin' Cajun & also replacing the V1 preamp tube with a lower gain JAN 5751. The amp is single channel & since the Gain control adds volume as well as od/dist, I'll most likely go even lower gain for V1 in order to run the amp as clean as possible.
 

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Electric I said:
Thanks a ton for the clear & concise info, good sir.


The plan is to convert this amp into a low-wattage recording amp. Research has led me to believe that London Power's PS-Box is the way to go, along with their bias mod kit for maximum serviceability. I do plan on buying a tube amp book/dvd or two, so suggestions welcome.
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If you are going the power scaling route, and are just going to scale the power amp section, make sure that you add some type of limit control in addition to the power scale. I've used Power Scale equipped amps for the last few years and the limit control is mandatory IMO.
 

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Discussion Starter #5
Scottone said:
make sure that you add some type of limit control in addition to the power scale.
Yes indeed. Apparently the control you mention is what London Power refers to as their 'Drive Compensation' control & comes standard with the add-on box.

From the LP site:

" Some automatic limiting of output stage distortion occurs if more than just the output stage is Power Scaled. In this case, the Drive Compensation control can be wired like a conventional master volume for fine adjustment of compression/distortion effects. Scaling more circuitry allows single-control over power and loudness, but ultimately reduces the amount of distortion possible - the counter-intuitive result also obtained using a "variac".

The PS-Box notes outline all of these options, and the Drive Compensation control is provided as a loose part. "

Thanks for the input.
 

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Electric I said:
Yes indeed. Apparently the control you mention is what London Power refers to as their 'Drive Compensation' control & comes standard with the add-on box.

From the LP site:

" Some automatic limiting of output stage distortion occurs if more than just the output stage is Power Scaled. In this case, the Drive Compensation control can be wired like a conventional master volume for fine adjustment of compression/distortion effects. Scaling more circuitry allows single-control over power and loudness, but ultimately reduces the amount of distortion possible - the counter-intuitive result also obtained using a "variac".

The PS-Box notes outline all of these options, and the Drive Compensation control is provided as a loose part. "

Thanks for the input.
Actually, doesn't that amp already have a master volume? In some cases that will suffice as a "limit" control.
 

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Discussion Starter #7
Scottone said:
Actually, doesn't that amp already have a master volume? In some cases that will suffice as a "limit" control.
Right you are - it does have a master volume. Since the drive compensation control is included as a loose part of the ps-box kit anyway, maybe the thing to do is leave it out of the mix to begin with & install if needed (?)
 

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Electric I said:
Right you are - it does have a master volume. Since the drive compensation control is included as a loose part of the ps-box kit anyway, maybe the thing to do is leave it out of the mix to begin with & install if needed (?)
That's what I would do. Let us know how things work out.
 
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